It depends on where you are trekking.If you are trekking in the Himalayan region, you should definitely wear thick jackets and jumpers, however, if you are trekking in the hilly area or mountainous area where it does not snow, you can be ok with normal clothes.

I had gone for a mountaineering course in Himachal Pradesh two summers ago. Even during the hot months of May the temperature can go below freezing during the night. It is best to wear thick jackets if you are not used to cold conditions. We had trekked up to an altitude of 15000 feet and at that height you absolutely need warm clothes due to snowstorms and cold winds.

I would recommend always at least taking warm clothing in the Himalayas even if you don't need them. Even at relatively low altitude you could become lost or one of your party may be injured and it can get considerably colder at night. I would certainly recommend a guided tour or going with an experienced local guide unless you are an experienced trekker as you never know when trouble may occur, and with good preparation for your trek you should be ok in almost any circumstances.